The inventor of the present invention has proposed high efficiency controlled cooling for maintaining an inside of a storage at around a predetermined temperature by using an inverter compressor (see Patent Document 1). The controlled cooling will be explained specifically with reference to FIG. 8. A control range is defined by an upper limit temperature TH that is a predetermined temperature higher than a target inside temperature To and a lower limit temperature TL that is a predetermined temperature lower than the target inside temperature To. A cooling characteristic (or a target temperature curve) that show variations in target temperature drop over time are predetermined for the control range. While the inverter compressor is driven in the control range, an actual temperature drop is calculated based on a detected inside temperature at every determined sampling time. The actual temperature drop is then compared with the target temperature drop contained in data on the above temperature curve at the inside temperature. If the actual temperature drop is smaller than the target temperature drop, the inverter compressor is controlled to increase the speed. If it is larger, the inverter compressor is controlled to decrease the speed. The inside temperature is controlled along the target temperature curve through the compressor speed control. Namely, the inside temperature is gradually decreased over a prolonged period of time in comparison to that in a pull-down range.
With this control, continuous turn-on time of the compressor greatly extends. Namely, switching between on and off is greatly reduced and the compressor is driven at a low rotation speed. Therefore, high efficiency and energy saving can be achieved. [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-121341